Interpretive Summary: Many chemicals produced by plants are used to ward off insects or mammals that eat them (herbivory). These same chemicals are also recognized as impacting (either positively or negatively), human physiology. How will a changing climate, most notably the documented increase in global temperature and carbon dioxide, alter the production of these chemicals? To understand this, we looked at three different chemicals, atropine, scopolamine and nicotine from two different plant species, jimson weed and tobacco. We used carbon dioxide (CO2)values that corresponded to the concentrations in the earth=s atmosphere that existed at the beginning of the 20th century, current values, and those projected for the year 2050 and temperature values projected for the next 20-40 years. We report here that any increase in CO2 resulted in a significant increase in leaf area and growth, while increased temperature resulted in earlier plant development and greater leaf area for both plant species. We also determined that increasing CO2 reduced the concentration of nicotine in tobacco; but had no effect on atropine, and increased the concentration of scopolamine in jimson weed. Conversely, temperature had no effect on nicotine or scopolamine concentration, but significantly increased the concentration and amounts of atropine per plant. However, because of the stimulatory effect of CO2 on growth, all three secondary compounds increased on a per plant basis (at either temperature) in both species. Overall, these data suggest that increases in CO2 and temperature associated with global climate change may have significant effects not only with respect to herbivory, but on the production of secondary compounds by plants that may impact human physiology. Data from this study will therefore be of benefit to entomologists, pharmacists, biochemists and ethnobotanists.